The production of polysilicon chunk materials via the decomposition of a gaseous precursor compound on a slim rod substrate is a well-known, widely used process commonly referred to as the “Siemens process.” The Siemens process is a combined decomposition/deposition process that comprises: (1) a heated rod or rods (appropriate substrates) covered by a suitable enclosure to allow high temperature, air-tight operation; (2) a system to feed the precursor material or compound of desired composition without contamination; (3) heating the enclosed rods to a desired temperature under appropriate environment; (4) decomposing the precursor material preferentially on the heated surface of the rods/substrate by distributing the gas appropriately near the growing rod vicinity; (5) recovery or disposal of byproduct or gases; and (6) recovery of product without contaminating the product.
In typical Siemens processes and reactors, the reactant gas is fed to the rods from a single port/nozzle resulting in uneven growth. Such uneven gas distribution over the length of the rod further promotes heavy homogeneous nucleation. Such uneven growth and homogeneous nucleation promote eventual reactor failure. Moreover, the rods within typical Siemens process reactors are not individually isolated. That makes the distribution of gas very difficult along the length of the rod that is growing. As a result, homogeneous nucleation, lower conversion, higher by-products, and uneven growth on the rods is further promoted by uneven radiant heat between the rods and gas precursor distribution.
According to known processes, elemental pure silicon is obtained in the Siemens type reactor, in the form of cylindrical rods of high purity by decomposing silicon halides from the distributed gas phase at a hot surface of the pure and purified silicon filament, the preferred halides being the chlorides, silicon tetrachloride and trichlorosilane. These compounds become increasingly unstable at temperatures above about 800° C. and decompose. Heterogeneous nucleation, hence silicon deposition, starts at about 800° C. and extends to the melting point of silicon at 1420° C. Since the deposition is beneficial only on the substrate, the inner walls of the decomposition chamber must not be exposed to the hot gasses so that there is no waste of valuable reactant gas
A further issue with the cooled wall reactor is the thermophoretic deposition of powder particles on the reactor walls. Such deposition is generally weak resulting in the multiple recirculation of the particles in the gas stream. This deposited powder eventually gets loose and collapse into the reactor, causing premature failure. That is why circulation and distribution of the reactant gas is very important.
The silicon halides used most frequently for the preparation of high purity silicon are silicon tetrachloride and trichlorosilane. These halides will undergo pyrolysis when in contact with the hot surface and deposit elemental silicon. To obtain reasonable and economical yields, however, an excess of hydrogen gas is added to the silicon halide vapor reaction feed gas. Because of its proportionally higher silicon content per unit weight and comparatively lower deposition temperature (i.e., faster kinetics), trichlorosilane will deposit more silicon than silicon tetrachloride and is therefore the preferred material for the Siemens' process for the preparation of polycrystalline silicon. Silicon halides with less than three chlorine atoms, such as SiH2Cl2 and SiH3Cl, in particular, deposit much more silicon per mole of silicon halide consumed in the reaction but are impractical because they are not readily available and thus less desirable economically. In any case, the yield is not more than about 20% and by-product gases are very difficult to handle.
Another approach to improved deposition rates is to use mixtures of silane and hydrogen where fast kinetics and lower temperatures assist faster deposition and better conversion. For example, silane (SiH4) offers itself as an effective silicon precursor and having no chlorine in the molecule improves the silicon to hydrogen ratios of silicon reaction gas mixtures. Silane decomposes above about 400° C. forming silicon and hydrogen. The byproducts formed are silane and hydrogen which may be readily recycled. The higher deposition rates and faster kinetics may require better distribution of the gas otherwise reactor will not work properly. Again, faster kinetics means faster depletion of gas resulting in uneven deposition unless the fresh gas is distributed evenly along the growth of the rod.